Atrial fibrillation (AF) in adult patients with congenital heart disease (ACHD) may appear early, depending on individual characteristics. The goals of this study were to investigate the epidemiological spectrum of AF in the entire cohort of ACHD and compare it with that in the general population. A retrospective study was performed in the nationwide cohort 2000-2014 with AF onset during 2003-2014. In the cohort of ACHD, 2350 patients had AF; the incidence increased with age, plateauing around age 70. In patients aged 25-29, 45-49, 65-69, 75-79, and ≥80 years, the annual incidence was 1.3, 7.9, 20.6, 23.7, and 21.4/1000 per year, respectively. In the general population without CHD, 347,979 patients had AF; the annual incidence was <1/1000 per year in those aged <55 years but increased steadily with age (3.6, 8.6, and 14.2/1000 per year in aged 65-69, 75-79, and ≥80 years, respectively). Compared with individuals without ACHD, ACHD patients aged <50 years and those aged both 50-54 and 55-59 years exhibited a 20-fold and 10-fold higher incidence of AF, respectively. Patients with complex congenital heart disease and Ebstein's anomaly had the highest risk of AF (cumulative risk >10% by age 50 and >20% by age 60), followed by those with tetralogy of Fallot, tricuspid atresia, endocardial cushion defect, and secundum atrial septal defect (cumulative risk >5% by age 50 and >10% by age60). Compared with individuals without ACHD, AF in patients with ACHD likely appeared 30 years earlier, with a 10- to 20-fold higher incidence plateauing around age 70. Yet, incidence in individuals without ACHD continued to increase. AF burden in patients with ACHD is not expected to increase in a never-ending way.